Zimmer Holdings Inc said it received a warning letter from the U.S. Food and Drug Administration citing problems with manufacturing and testing processes for artificial hip devices made at the company's Ponce, Puerto Rico, plant. The warning letter, dated September 19, does not restrict production or shipment of the hip devices, called Trilogy Acetabular Systems, or require withdrawal of any products from the market, Zimmer said in a regulatory filing.The FDA said Zimmer failed to implement a testing mechanism to demonstrate the products met certain design specifications and had not validated the effects of manufacturing operations on products that incorporate metallic spikes.